1. Field of the Invention
The present invention relates generally to the control of an internal combustion engine powered by diesel fuel, and more specifically to reducing transient NOx generation produced by such a vehicle.
2. Background of Invention
Controlling NOx emissions in diesel engines has posed significant challenges to the automotive industry. While emission control devices, such as NOx catalysts, can be used, these devices may be insufficient to meet ever-increasing emission standards.
A method to reduce NOx in diesel engines is the use of exhaust gas recirculation (EGR). EGR works very well during steady, or near steady, engine operation because there is typically an abundance of air at those conditions.
However, under transient engine operation in which a vehicle is required to accelerate, EGR can limit the performance of the vehicle by reducing the amount of airflow through the engine. EGR reduces airflow by displacing air in the combustion chamber, heating up the intake charge, and redirecting exhaust gas that would normally go through the turbocharger to the intake manifold. This last effect reduces the energy flow through the turbine, thus diminishing the engine's ability to create boost. This phenomenon can be dubbed the “EGR-Boost tradeoff”.
Typically, conventional diesel systems suspend the use of EGR in order to accelerate aggressively because of the aforementioned reasons. However, without EGR or some other means to reduce NOx emissions, NOx (concentration) increase dramatically. This comes at a time when the air mass flow rate is very high, causing NOx production to spike. Thus, it is necessary to find some means to control NOx emissions during hard accelerations, either by finding another strategy by which NOx can be reduced or by finding a way overcome the enable the presence of EGR without diminishing the flow of exhaust gas through the compressive device.
Another approach to further reduce emissions is to use split injection of fuel with a very early injection of fuel. Such an approach is described in U.S. Pat. Nos. 6,276,334 and 5,979,398.
The inventors herein, however, have recognized a disadvantage with the above approach. In particular, using multiple injections decreases fuel economy. In other words, while the above methods can help reduce NOx, there are conditions where the disadvantages of this mode of operation outweigh its benefits.
Also, other multiple injection strategies are known. These various other methods using multiple injections combine the main injection with one or more of the other injection events. Post injection is a strategy that is used to reduce soot emissions by injecting a small amount of fuel near the end of combustion. The post injection increases in-cylinder mixing and temperature, thus increasing soot oxidation rates. Split main injection enhances fuel-air mixing by allowing air to mix in between fuel pulses, resulting in lower soot. However, neither of these strategies is known to lower NOx emissions. Pilot injection involves the injection of a very small quantity of fuel into the combustion chamber only slightly before the start of the main fuel injection. While pilot injection has been shown to lower NOx from the engine, it also causes a substantial increase in soot emissions, which becomes even greater without very precise control of small fuel quantities. As such, these various other multiple injection strategies provide little guidance to the problem at hand.